Traditional methods of wastewater treatment involve bringing wastewater streams into contact with bacteria in an aerobic and/or anaerobic type process in what is known as activated sludge treatment. These bacteria consume parts of the substrate material or waste contained in the wastewater, which are typically organic compounds containing carbon, nitrogen, phosphorus, sulfur, and complex derivations thereof. Typically, a portion of the waste is consumed to further the metabolism of the bacterial cells or maintain the physiological functioning of the bacterial cells. In addition, a portion of the waste is consumed as part of the process of synthesis of new bacterial cells. The activated sludge treatment process yields a certain amount of sludge and associated solids which must be continuously removed from the treatment basin to maintain the steady state sludge balance which is critical to the effective functioning of the activated sludge treatment system. It is also important that when treating wastewater that the operator maintains the appropriate carbon, nitrogen and phosphorous (C/N/P) ratios or nutrient levels within the wastewater system. This is of particular concern where there may be regulated nutrient limitations such as many industrial wastewater treatment systems or in applications where biological phosphorus removal is required.
In order to maintain waste removal capacity of the treatment plant at steady state or other desired level, it is important to control the accumulation of new bacterial cells within the activated sludge treatment process. An excessive accumulation of new bacterial cells in excess of what is required for the treatment of the waste at or near steady state results in a deviation from optimal design considerations such as the Food to Mass ratio (F/M) or the Mixed Liquor Suspended Solids (MLSS) which are required to be within certain optimal ranges to allow for effective organic treatment and aeration efficiency. Thus, the excess biosolids must be continuously removed during the activated sludge treatment process.
Existing methods for dealing with the removal of sludge includes transporting the sludge to landfills, utilization of sludge for land application or agricultural purposes, and incineration of the sludge. Most sludge disposal operations require some prior treatment of the sludge; a process known in the art as solids handling. Solids handling processes are often costly and time consuming operations and typically involve one or more of the following steps: concentration of the sludge in a thickener, usually requiring the use of polymers; digestion of the sludge in order to stabilize the bacteria and to further reduce the volume and pathogen content of the sludge; dewatering of the sludge to reach approximately 15-25% solids content, which involves the passage of the sludge through centrifuges or other solid-liquid separation type devices; storage of the sludge; and transportation to sites for landfill, land application by farmers, or other end use.
It is estimated that the costs associated with solids handling and disposal processes can be between 20-60% of the total operating costs associated with the overall wastewater treatment process. Due to the cost and time associated with solids handling and disposal, it is beneficial to minimize the amount of excess sludge produced in the wastewater treatment process.
In conventional activated sludge treatment systems and methods, the use of ozone in addition to oxygen for the treatment of sludge has been reported. More particularly, ozone treatment of sludge has been reported in combination with mechanical agitators and/or pumps providing the motive mixing. The sludge-ozone contact typically occurs in a continuously stirred tank reaction (CSTR) mode, and lysis breaching of the integrity of the cell wall results as a consequence of the strong oxidizing action of ozone on the cell walls. Lysis leads to the release of the substrate rich cellular content of the bacterial cells. In this way, the solid cells which would otherwise have been discharged as excess sludge are lysed, and by so doing, are transformed to substrate which can then be consumed by bacteria in the treatment basin.
Different types of reactor systems are known for ozone treatment of sludge, including a CSTR, a higher selective plug flow reactor and a batch reactor system. The major difference between the different reactor modes lies fundamentally in: (i) the average amount of time that sludge stays within the reaction space, also known as the residence time; (ii) the interaction between reacting ‘parcels’ e.g., significant back-mixing in the CSTR and batch reactor systems whereas very limited back-mixing in the plug flow reactor system; and (iii) the yield or ozone dosing levels required to eliminate a volume of sludge.